1. Field of the Invention
This invention relates to a camera having a rotary member arranged to detect the movement of film.
2. Description of the Related Art
In imprinting data on the picture plane of film with a data imprinting device of a camera, it has been practiced to imprint the data from the base face of the film, i.e., from on the side of the back cover of the camera. However, characters imprinted from this side of the film tend to smear as the imprinting light is applied through the base layer of the film. Further, since the film base is colored, it is at times difficult to obtain a clear imprint because of an adverse effect of the color of light emitted by a light emitting element or the color of an object image around the data imprinted.
It is known to solve this problem by imprinting data from on the side of the emulsion layer of the film, i.e., from on the side of a photo-taking lens. In the case of a single-lens reflex camera, however, a focal plane shutter unit occupies a major portion of the area of an aperture. The data imprinting device must be arranged in such a way as to avoid hindering the focal plane shutter unit as well as the photo-taking optical path. Further, in order to imprint data in association with a film winding action to be performed after each shot, the data imprinting device must be disposed as close to the aperture as possible. It is, however, very difficult to meet these requirements, particularly in cases where a sprocket is arranged to engage the perforation of the film.
In the conventional camera, the sprocket is arranged to be rotated by the film by engaging the perforation of the film. An encoder is secured to the shaft of the sprocket. An electric digital signal is generated by detecting the degree of rotation of the encoder by means of a photo-sensor. The amount of film feeding is detected by counting the pulses of this signal by a pulse counter.
However, the film perforation and the teeth of the sprocket must be arranged to have a certain fixed tolerance for their engagement. It is, therefore, hardly possible to detect the film feeding amount with a high degree of accuracy by the above-stated known sprocket type method for detecting a film feeding amount.
Some of the latest cameras is provided with a character information imprinting device of a dot matrix type which is arranged to be driven in association with a film feeding action by an electronic method. However, in accurately imprinting the character information or the like in a prescribed part of a film frame plane requires use of a film feeding amount detecting means of a high degree of precision. Since the film feeding amount cannot be accurately detected by the conventional sprocket type film feeding amount detecting means, a friction-driven roller is being employed as the film feeding amount detecting means in place of the sprocket.
However, the use of a friction-driven roller which is arranged to be rotated and driven by friction with the film for detecting the amount of film feeding in place of the conventional sprocket brings about the following problem: Referring to FIGS. 21(a) and 21(b), in this instance, the film F which has passed the aperture has its traveling direction bent along the outer circumferential surface of the friction-driven roller R while it is moving toward a spool S. Therefore, a compressing force is exerted on the inner film surface f1 which is in contact with the outer circumferential surface of the roller R. This causes the inner film surface f1 to shrink and causes a tensile force to act on the outer film surface f2 to elongate it. As a result, an error arises between an actual film feeding amount and a film feeding amount detected through the rotation angle of the roller R. Hence, the film feeding amount cannot be accurately detected with the roller in the above-stated manner.
In FIGS. 21(a) and 21(b), a reference symbol r1 denotes a pinch roller which is provided for pushing the film F against the outer circumferential surface of the friction-driven roller R. A film retaining roller r2 is provided for pushing the film F against the outer circumferential surface of the spool S. A roller support lever L carries both the rollers r1 and r2. A spool chamber SR is formed in a camera body CB. An arrow "a" indicates the direction in which the film F travels. Another arrow "b" indicates the rotating direction of the friction-driven roller R.